Adhesive tapes and sheets are generally produced by coating a paper, nonwoven cloth, synthetic resin film or metal foil support or base with an adhesive and drying the adhesive coating layer.
As the adhesive mentioned above, either a solvent-type adhesive or an aqueous dispersion type emulsion adhesive is generally employed. However, since aqueous emulsion adhesives containing water as a dispersing medium do not contain any organic solvent, they are used on more and more occasions of late for the protection of health, prevention of atmospheric pollution and fire hazards, and conservation of resources.
The conventional acrylic emulsion adhesives of this type are generally produced by emulsion-polymerizing a monomeric mixture comprising a (meth)acrylic acid ester, as a predominant component, and one or more other copolymerizable vinyl monomers together with an emulsifier in aqueous medium. It has been reported that a high-solid acrylic polymer emulsion can be produced with good stability by using a surfactant mixture of an anionic surfactant and a nonionic surfactant in conducting such an emulsion polymerization reaction (Japanese Kokai Publication S-63-27572).
However, the above technology has the disadvantage that said surfactant mixture must be added in a comparatively large proportion of 2-15 parts by weight relative to 100 parts by weight of the monomer component with the result that the water resistance of the adhesive is decreased. Thus, the adhesive not only tends to suffer a loss of adhesion but undergoes opacification or whitening when immersed in water or in a high temperature, high humidity environment.
Particularly when the above emulsion polymer adhesive is used in the manufacture of an adhesive sheet for surface protection (surfacer) which, in use, is laminated onto a poster, paper label or the like for protecting the printed characters and design against external influences such as water, sunlight, etc., the characters and design tend to become invisible because of said whitening and, moreover, the surfacer may become detached from the poster or the like due to reduced adhesion.
Developed to obviate these disadvantages of the prior art, this invention has for its object to provide a method of producing an acrylic emulsion adhesive having improved water resistance without compromise in adhesion and a low-viscosity, high-solid feature with good polymerization stability, an acrylic emulsion adhesive having such desirable properties, and a surfacer comprising said adhesive on a base tape or sheet.